Solid electrolyte ionic or mixed ionic-electronic conductors that can rapidly transport oxygen ions have a significant potential for use in air separation. Membranes made of such materials transport only oxygen ions and, therefore, have an infinite selectivity for the permeation of oxygen relative to all other species. This property is of particular advantage in the production of oxygen, since the oxygen product is inherently pure. Conversely, solid electrolyte ion transport materials may also be used to remove oxygen from an air stream to produce an oxygen-free "nitrogen" product.
Air is a mixture of gases which may contain varying amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. When a basic solid electrolyte ion transport process is used with air as the feed gas, the minor impurities in the feed air stream (for example, argon, carbon dioxide, water and trace hydrocarbons) are retained in the "nitrogen" retentate.
Therefore, solid electrolyte ionic conductors that transport only oxygen ions appear to be attractive for the separation of oxygen from gas mixtures such as air. These materials, which can be purely ionic conductors or mixed conductors capable of transporting oxygen ions and electrons, are particularly attractive because of their infinite selectivity for oxygen over all other gases. One consequence of this is that the oxygen produced by the solid electrolyte ion transport separator is of ultra high purity (UHP). Ultra high purity oxygen, however, is highly reactive, especially at elevated pressures and temperatures. Thus, the handling of ultra high purity oxygen for transport (for example, with piping), heat transfer, etc., tends to be expensive and often requires the use of special materials. Furthermore, most of the current applications of oxygen require purities of only 90-99% and there is seldom any benefit in increasing the purity to UHP levels and dealing with the increased difficulties of handling UHP oxygen.
In traditional non-cryogenic air separation processes, the concentration of oxygen is kept low and is purified in increments and there is no need to either produce or dilute UHP oxygen. In contrast, the oxygen produced by cryogenic distillation can be very pure (approximately 99.5% pure) and special materials and procedures are typically employed because of the enhanced reactivity of UHP oxygen. The materials that can be used safely in cryogenic processes, therefore, depends on the oxygen concentration encountered. As the oxygen concentration becomes higher, there are only a few special materials that are safe to use. The requirements can be even more severe in solid electrolyte ion transport processes which produce UHP oxygen and which must necessarily operate at a high temperature which usually increases the rate of reaction of oxygen with the materials it is allowed to contact.
Extensive studies have been made by producers of oxygen by cryogenic distillation to determine which materials are suitable for use in liquids or gases containing high concentrations of oxygen. These studies, which are relevant to the ultra high purity oxygen produced by solid electrolyte ion transport processes, show a very strong dependency of reactivity on the oxygen concentration and the pressure. As the oxygen concentration or the pressure increases, materials are more prone to be oxidized and combusted (that is, oxidized rapidly) and there are fewer materials that can be safely used in such an application. For example, FIG. 3 is a graph showing the ignition-combustion behavior at room temperature of Haynes alloy No. 25, a cobalt-based alloy with high corrosion resistance, at 80%, 90%, and 99.7% oxygen concentrations. It can be seen from the graph that the burning of the Haynes alloy No. 25 sample is promoted dramatically by higher oxygen concentrations. FIG. 4 is a graph showing the flammability data at room temperature for a 3.175 mm (0.125 in) diameter rods of HASTELLOY.RTM. alloy C-22.TM., a nickel-chromium-molybdenum alloy with high corrosion resistance, where combustion only occurred at high oxygen concentration and elevated pressure. Unfortunately, despite the information that can be gleaned from these tests, there are few tests that have been made in the temperature range where solid electrolyte ion transport devices are used, which is generally in excess of 600 C. It is likely that such increased temperatures would make the reactivity problem even more severe. It is evident, however, that decreasing the oxygen concentration from pure oxygen to a lower value decreases the susceptibility of materials in contact with the oxygen to oxidization or combustion.
There are now two types of solid electrolyte ion transport membranes under development: ionic conductors that conduct only ions through the membrane and mixed conductors that conduct both ions and electrons through the membrane. An ion transport membrane exhibiting such mixed conduction characteristics can transport oxygen when subjected to a differential partial pressure of oxygen across the membrane without the need for an applied electric field or external electrodes which would be necessary with the ionic conductors. As used herein, the terms "solid electrolyte ionic conductor", "solid electrolyte ion transport system", or simply "solid electrolyte" or "ion transport membrane" is used to designate either a system using ionic-type system or a mixed conductor-type system unless otherwise specified.
Solid electrolyte ion transport technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, which is hereby incorporated by reference to more fully describe the state of the art.
In the absence of a purge stream, the "permeate" stream that carries the oxygen away from the ion transport membrane is "pure" oxygen. For mixed conduction membranes both the feed and the retentate streams must be at a high pressure (or the "permeate" stream at a very low pressure) to create a driving force for the oxygen transport. While such an unpurged membrane is attractive for the removal of larger quantities of oxygen from inert gas streams, the oxygen recovery is limited by pressures that can be applied.
The inventors are unaware of any prior art describing the dilution of oxygen produced by a solid electrolyte ion transport process. The following references concern the oxidation resistance of materials as a function of pressure and oxygen concentration: R. Zawierucha, K. McIlroy, and J. F. Million, Flammability of Selected Heat Resistant Alloys in Oxygen Gas Mixtures, Proceedings of the 2nd International Conference on Heat-Resistant Materials, Gatlinburg, Tenn., September 1995, pp. 97-103; and R. Zawierucha, R. F. Drnevich, D. E. White and K. McIlroy, Materials and Systems Considerations for Applications Involving Oxygen Enriched Atmospheres, Presented at the ASME Winter Annual Meeting, New Orleans, La., December 1993.
Chen et al., U.S. Pat. No. Re. 34,595 (reissue of U.S. Pat. No. 5,035,726), entitled Process for Removing Oxygen and Nitrogen from Crude Argon, relates to the use of electrically-driven solid electrolyte membranes for the removal of low levels of oxygen from crude argon gas streams. Chen et al. estimate the electrical power needed for several examples of multistage processes and also mention the possibility of using mixed conductor membranes operated by maintaining an oxygen pressure on the feed side. Chen et al. further teach that oxygen exiting from the permeate side of an electrically-driven ionic membrane may either be removed as a pure oxygen stream or mixed with a suitable "sweep" gas such as nitrogen.
Mazanec et al., U.S. Pat. No. 5,160,713 entitled Process for Separating Oxygen from an Oxygen-Containing Gas by Using a Bi-Containing Mixed Metal Oxide Membrane, relates to an oxygen separation process employing a bismuth-containing mixed metal oxide membrane which generally provides that the separated oxygen can be collected for recovery or reacted with an oxygen-consuming substance. The oxygen-depleted retentate is apparently discarded.
Mazanec et al., U.S. Pat. No. 5,306,411, entitled Solid Multi-Component Membranes, Electrochemical Reactor Components, Electrochemical Reactors and Use of Membranes, Reactor Components, and Reactor for Oxidation Reactions, relates to a number of uses of a solid electrolyte membrane in an electrochemical reactor. It is mentioned that nitrous oxides and sulfur oxides in flue or exhaust gases can be converted into nitrogen gas and elemental sulfur, respectively, and that a reactant gas such as light hydrocarbon gas can be mixed with an inert diluent gas which does not interfere with the desired reaction, although the reason for providing such a mixture is not stated. Neither of the Mazanec et al. patents cited disclose processes to produce a purified product from an oxygen-containing gas stream.